concubine vs mistress

concubine and mistress both are nouns.

Word	Noun	Adjective	Verb	Adverb
concubine	Yes	No	No	No
mistress	Yes	No	No	No

As nouns, mistress is a hypernym of concubine; that is, mistress is a word with a broader meaning than concubine:

concubine: a woman who cohabits with an important man
mistress: an adulterous woman; a woman who has an ongoing extramarital sexual relationship with a man

Other hypernyms of concubine include fancy woman, kept woman.

concubine (noun)	mistress (noun)
a woman who cohabits with an important man	a woman master who directs the work of others
	an adulterous woman; a woman who has an ongoing extramarital sexual relationship with a man
	a woman schoolteacher (especially one regarded as strict)